The coefficient of determination, r, indicates the strength and direction of the linear relationship between two variables. It ranges from -1 to 1, where a value of 1 indicates a perfect positive linear relationship, 0 indicates no linear relationship, and -1 indicates a perfect negative linear relationship. It is a measure of how well the data points fit a straight line, with higher values indicating a stronger linear relationship. Therefore, the correct answer is "the linear relationship between two variables."